E164.org lists regular PSTN telephone numbers and maps them to VoIP (Voice over IP) addresses such as those provided by Free World Dialup, Broadband Phone Providers like Vonage, or even people who run their own Asterisk system
SIP Switch is an online call management application, enabling you to use multiple SIP providers services from one piece of hardware and to manage all your calls on the fly: hang up, transfer, forward ...